Section 4

4. But there is the question of the linked light that must
relate the visual organ to its object.

Now, firstly: since the intervening air is not necessary- unless
in the purely accidental sense that air may be necessary to
light- the
light that acts as intermediate in vision will be unmodified: vision
depends upon no modification whatever. This one intermediate, light,
would seem to be necessary, but, unless light is corporeal, no
intervening body is requisite: and we must remember that
intervenient and borrowed light is essential not to seeing in
general but to distant vision; the question whether light absolutely
requires the presence of air we will discuss later. For the present
one matter must occupy us:

If, in the act of vision, that linked light becomes ensouled, if
the soul or mind permeates it and enters into union with it, as it
does in its more inward acts such as understanding- which is what
vision really is- then the intervening light is not a necessity: the
process of seeing will be like that of touch; the visual faculty of
the soul will perceive by the fact of having entered into the light;
all that intervenes remains unaffected, serving simply as the field
over which the vision ranges.

This brings up the question whether the sight is made active
over its field by the sheer presence of a distance spread before it,
or by the presence of a body of some kind within that distance.

If by the presence of such a body, then there will be vision
though there be no intervenient; if the intervenient is the sole
attractive agent, then we are forced to think of the visible
object as
being a Kind utterly without energy, performing no act. But so
inactive a body cannot be: touch tells us that, for it does
not merely
announce that something is by and is touched: it is acted upon by
the object so that it reports distinguishing qualities in it,
qualities so effective that even at a distance touch itself would
register them but for the accidental that it demands proximity.

We catch the heat of a fire just as soon as the intervening air
does; no need to wait for it to be warmed: the denser body, in fact,
takes in more warmth than the air has to give; in other
words, the air
transmits the heat but is not the source of our warmth.

When on the one side, that of the object, there is the power in
any degree of an outgoing act, and on the other, that of the sight,
the capability of being acted upon, surely the object needs no
medium through which to be effective upon what it is fully
equipped to
affect: this would be needing not a help but a hindrance.

Or, again, consider the Dawn: there is no need that the light
first flood the air and then come to us; the event is simultaneous
to both: often, in fact, we see [in the distance] when the light is
not as yet round our eyes at all but very far off, before, that is,
the air has been acted upon: here we have vision without any
modified intervenient, vision before the organ has received the
light with which it is to be linked.

It is difficult to reconcile with this theory the fact of seeing
stars or any fire by night.

If [as by the theory of an intervenient] the percipient mind or
soul remains within itself and needs the light only as one might
need a stick in the hand to touch something at a distance, then the
perception will be a sort of tussle: the light must be conceived as
something thrusting, something aimed at a mark, and similarly, the
object, considered as an illuminated thing, must be conceived to be
resistant; for this is the normal process in the case of contact by
the agency of an intervenient.

Besides, even on this explanation, the mind must have previously
been in contact with the object in the entire absence of
intervenient;
only if that has happened could contact through an intervenient
bring knowledge, a knowledge by way of memory, and, even more
emphatically, by way of reasoned comparison [ending in
identification]: but this process of memory and comparison
is excluded
by the theory of first knowledge through the agency of a medium.

Finally, we may be told that the impinging light is modified by
the thing to be seen and so becomes able to present something
perceptible before the visual organ; but this simply brings
us back to
the theory of an intervenient changed midway by the object, an
explanation whose difficulties we have already indicated.